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PCB3063: Chapter 5

by: Brittany Woody

PCB3063: Chapter 5 PCB3603

Brittany Woody

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These are notes from the powerpoint and lecture for Chapter 5. They are supplemented with information from the textbook. Concepts of Genetics 11th Edition
Dr. W. Brad Barbazuk
Class Notes
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This 3 page Class Notes was uploaded by Brittany Woody on Tuesday September 20, 2016. The Class Notes belongs to PCB3603 at University of Florida taught by Dr. W. Brad Barbazuk in Fall 2016. Since its upload, it has received 18 views. For similar materials see Genetics in Genetics at University of Florida.


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Date Created: 09/20/16
Sunday, September 11, 2016 Chapter 5 Linkage and Chromosome Mapping in Eukaryotes - Un-linked genes will assort independently during meiosis - Gene loci that reside on the same chromosome are considered linked – and will co- segregate; assuming complete linkage, all gametes will be non-crossover or parental gametes - During the first meiotic prophase a reciprocal exchange of genetic material can occur; this crossing-over phenomena occurs at the site of chiasmata; this process can result in the reshuffling or recombination of the alleles found on the two homologues; this results in creation of homologous chromosomes with allele combinations that differ from the parental homologs – this results in recombinant gametes - No recombination: two genes on a single pair of homologs; no exchange occurs - Recombination: two genes on a single pair of homologs; exchange occurs between two non sister chromosomes - Genes on the same chromosome are part of a linkage group; the number of linkage groups should correspond to the haploid number of chromosomes; the percentage of offspring resulting from recombinant gametes depends on the distance between the two genes on the chromosome - The ratio of phenotypes produced from a cross between two organisms heterozygous at linked loci is the linkage ratio - Recombination events will occur between many linked genes; based on work with flies mutant for white eyes and yellow bodies, Morgan hypothesized: • crossing over uncouples alleles resulting in recombinant gametes • linked genes are arranged in linear sequence • variable frequency of exchange occurs between any two genes as a result of their distance - One map unit is defined as 1 percent recombination between two genes on a chromosome - Map units are often called centimorgan (cM) and are relative distances, not exact ones 1 Sunday, September 11, 2016 - When a single crossover occurs between 2 non-sister chromatids, the other 2 chromatids of the tetrad are unchanged; the percentage of tetrads involved in a exchange between two genes is twice the percentage of recombinant gametes produced; even if a single crossover occurs in 100% of tetrads, only 50% of the gametes are recombinant - When two linked genes are more than 50 map units apart, a crossover theoretically can be expected to occur between them in 100 percent of the tetrads - Single crossovers can be used to determine the distance between two linked genes, but double crossovers (DCOs) can be used to determine the order of three genes on the chromosome; the expected frequency of double-crossover gametes is much lower than that of either single-crossover gamete class - Three-point mapping is a mapping cross that allows the mapping of 3 or more linked genes in a single cross; in a 3- point mapping, one parent (usually female) is heterozygous for all obi under consideration (these are the gametes we are considering) - The other parent (usually male) is homozygous recessive for all loci under consideration; this is much like a testcross • need to look at adequate number of progeny • need to make sure that genotype is implicit in phenotype; this is how we evaluate the outcome of every meiosis in the heterozygote - The non-crossover F2 phenotypes occur in the greatest proportion of offspring - The double-crossover phenotype occur in the smallest proportion - The distance between two genes in a three-point cross is equal to the percentage of all detectable exchanges occurring between them and includes all single and double crossovers - The expected frequency of multiple exchanges between two genes can be predicted from the distance between them; the coefficient of coincidence (C) is the observed number of DCOs divided by the expected numbers of DCOs - Interference reduces the expected number of multiple crossovers when a crossover event in one region of the chromosome inhibits a second event near; if interference is complete, no DCO will occur - C is the coefficient of coincidence= Observed DCOs/ Expected DCOs 2 Sunday, September 11, 2016 - Interference is quantified by I; I= 1- C • positive interference: fewer DCOs occurred than expected (I is greater than 0) • negative interference: more DCOs occurred than expected (I is less than 0) - As the distance between two genes increases, mapping experiments become less accurate; the discrepancy results primarily from multiple exchanges that are predicted to occur between the two genes but that are not recovered during experimental mapping - Elliptocytosis: shape of erythrocytes is an oval - Log of odds (lod) favoring linkage score analysis relies on probability calculations to demonstrate linkage between two genes in organisms in which linkage analysis relies primarily on pedigrees; LOD score accuracy is limited by the extent of the pedigree - Somatic cell hybridization involves fusion of two cells in culture to form a single hybrid cell, called a heterokaryon - Upon continued culturing of the hybrid cell, chromosomes from one of the two parental species are gradually lost until only a few chromosomes of one species remain and most chromosome are from the other species, creating what is termed a synkaryon - A panel of cell lines, each containing just a few human chromosomes, can be used for sentence testing in which the presence or absence of a specific gene product is correlated with the presence or absence of each chromosome - DNA markers and higher resolution cell lines (radiation hybrids) provide a means to very dense maps - Earliest example of DNA markers are restriction fragment length polymorphisms (RFLPs); also called micro satellites or short tandem repeats - Variations in a single nucleotide is called single-nucleotide polymorphisms (SNPs) - Crossing over involves physical exchange between chromatids; mapping in maize using cytological markers established that crossing over involves a physical exchange of chromosome regions - Recombination between mitotic chromosomes are rare, <1% - Sister chromatid exchanges (SCEs) occur during mitosis but do not produce new allelic combinations 3


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